Some self-service terminals (SSTs), such as automated teller machines (ATMs), can receive banknotes deposited by a customer. Some anti-theft systems include automatic ink staining of banknotes when a banknote cassette is withdrawn, or otherwise accessed, by an unauthorized person. Such systems cause the cassette to discharge an ink stain onto the stack of notes contained within the cassette. This ink staining on the banknotes is highly visible and is designed to alert people who may receive a stained banknote that the banknote may have been stolen.
To avoid alerting people that a banknote is stolen, criminals may deposit stained banknotes into a bank account using an ATM so that no human is present to look at the deposited banknote.
In addition, banknotes may become stained accidentally, for example, through spillage of ink, coffee, or some other liquid.
Banknote issuing authorities (such as the European Central Bank) desire to remove stained banknotes from circulation (regardless of whether those banknotes were stained as a result of theft deterrence, or accidentally stained), so it is desirable for an ATM to be able to detect stained banknotes when such banknotes are presented to ATMs.
Although it is easy for a human to identify staining on a banknote, it is much more difficult for an automated system because a banknote can be presented in four different orientations, and using a single color of visible light to image banknotes may not be sufficient to detect the staining because the stain may be the same color as the light source.